This invention relates generally to relatively high-power lasers and, more particularly, to lasers of the free-electron type. In contrast to other laser types in which electrons may be bound to a single atom or molecule, or in which electrons may be free to move through the entire volume of semiconductor, the free-electron laser produces stimulated emission from a beam of free electrons in a vacuum.
Basically, in a free-electron laser, a beam of relativistic electrons, that is, electrons that have been accelerated to speeds comparable with the speed of light, is passed through a transverse and periodic magnetic field, known as a "wiggler," which results in periodic transverse movement of the electrons. Light is emitted in the direction of the electron beam as a result of the interaction between the electrons and the magnetic field, and is fed back through the wiggler by means of two opposed mirrors. Stimulated emission comes about through the interaction of the electromagnetic wave fed back and forth and the periodic magnetic structure.
An important property of the free-electron laser is that it is frequency-tunable over a relatively wide range. The wavelength .lambda. of light emission from the laser is directly proportional to the wavelength of the periodic magnetic wiggler field, and is inversely proportional to the square of .gamma., which is a measure of electron energy. The quantity .gamma. is the electron energy expressed as a multiple of the "at rest" energy of the electron. An electron at rest has a .gamma. value of unity, equivalent to the energy it possesses solely as a result of its mass. As the velocity of the electron is increased, its total energy increases, and may be usefully expressed in terms of its .gamma. value.
The velocity, v, of an electron may be expressed as a fraction, B of the speed of light, c, as follows: EQU .beta.=v/c=1-1/.gamma..sup.2
as the energy of the electron increases, indicated by larger values of .gamma., the speed approaches, but never reaches the speed of light.
In one experiment using a free-electron laser, an output wavelength of 10 microns (micrometers) was achieved with a wiggler wavelength of 3.5 cm and an energy of 25 Mev (million electron volts) to accelerate the electron beam. To obtain an optical output wavelength of 1.6 microns, an accelerating energy of 66 Mev had to be used, and to obtain an output wavelength of 0.5 micron, the accelerating energy level had to be raised to 117 Mev. In general to obtain a decrease in wavelength by a given factor, the energy must be increased by approximately the square root of that factor.
Therefore, shorter wavelengths can be obtained from a free-electron laser only at the expense of increased electron energy. Since free-electron lasers operate at relatively low efficiency levels and require expensive linear accelerators to produce the electron beam, it is often difficult to achieve laser oscillator action, which requires sufficient gain to exceed the losses in the optical cavity of the system. There is clearly a need for a free-electron laser that will require less accelerating energy for a given frequency, and the present invention is directed to this need.